1. Field of the Invention
The present invention generally concerns mass flow sensors, and more particularly, concerns mass flow sensors made of micro-electromechanical systems (MEMS) approach, and methods of manufacturing such mass flow sensors.
2. Description of the Related Art
Conventional technologies of mass flow sensors are still limited by the difficulties of limited ranges of flow rate measurement and the requirement to maintain a high level of heating power. Specifically, the commercially available mass flow sensors are commonly made of transducers that include heaters and temperature sensors. The heater and temperature sensors are commonly provided with resistance wires such as platinum wires on a ceramic substrate. The stream of flow when passing over the mass flow sensor, carries away the heat from the heater thus causes temperature variations. The temperature variations and distributions depend on the mass flow rate, e.g., the velocity and the material properties of the flow. Thus the temperature of the heater and temperature distributions as that measured by the temperature sensors around the heaters are then applied to calculate the mass flow rate.
Over the past few years, the emergence of micro-electromechanical system (MEMS) technology has enabled the fabrication of mass flow sensors directly on silicon. The MEMS flow sensor can be provided with small size, low power, and high reliability at low cost. With low power dissipation, MEMS mass flow sensors can be used for measuring explosive gas mixtures over low explosion limits without risks.
Thermal mass flow sensors can be classified into three basic categories: anemometers, calorimetric flow sensors, and time-of-flight sensors. Jiang et al disclose a micromachined anemometer type flow sensor comprising of a single element, which is heated and a measurement of heat loss is performed. Please referred to “F. Jiang, Y. C. Tai, C. M. Ho, and W. J. Li, “A Micromachined Polysilicon Hot-Wire Anemometer,” Digest Solid-State Sensors & Actuator Workshop, Hilton Head, S.C., pp. 264-267, 1994 for more details. This heat loss is dependent on the flow rate of the fluid. In general, this heat loss increases with the flow velocity, and the signal of an anemometer is proportional to the square root of the flow velocity. However, due to the variations and uncertainties of flow velocity, this measurement technique has limited accuracy when applied to measurements over a relative large range of flow rate measurement.
FIG. 1 shows a typical mass flow sensor device that generally includes a thin-film heating element and a pair of thin-film sensing resistors on a thin thermally-isolated membrane on the surface of a machined silicon substrate. As shown in FIG. 1, each pair of sensing resistors is symmetrically arranged with respect to the center heater at flow upstream and downstream, respectively. In the operation of the flow sensor, the heater element is energized to produce a temperature at the center that is considerably higher than ambient temperature. A temperature gradient thus is produced from the center heater to edges of the membrane area. The sensing resistors preferably made of materials, exhibiting a high resistance dependence on temperature, preferably a high thermal coefficient resistance (TCR) (e.g., 3000 ppm/° C.˜8300 ppm/° C.). Accordingly, the resistances of the sensing resistors are caused to change in proportional to the temperature change in the area of the membrane containing the sensing resistors.
In the operation of flow sensor, a moving fluid carries away heat in the direction of flow and as a result changes the temperature distribution around the heater. The sensing resistors located at the upstream and downstream of the heater then measure the temperature difference between upstream and downstream locations. A Wheatstone bridge circuit, in which a pair of downstream and upstream sensing resistors comprises two of its four branches, fetches the output signal. The output signal, which is a measure of temperature difference, is proportional to the flow velocity initially until a high flow velocity is reached where the temperature difference saturates and then decreases at higher flow velocity. As shown in FIG. 1, a reference resistor on silicon substrate is generally used to monitor the ambient temperature. A Wheatstone bridge circuit consisting of the heater and the reference resistor can be formed to achieve constant-temperature control.
Hariadi et al disclose a time-of-flight flow sensor fabricated on Silicon-On-Insulator (SOI) wafers. The pulse is fed to the fluid by a heater and a temperature sensor located downstream detects its delay. Please refer to “I. Hariadi, H.-K. Trieu, W. Mokwa, H. Vogt, “Integrated Flow Sensor with Monocrystalline Silicon Membrane Operating in Thermal Time-of-Flight Mode,” The 16th European Conference on Solid-State Transducers, Sep. 15-18, 2002, Prague, Czech Republic” for additional details. Measuring a flight time, the sensors provide data to calculate the velocity of the streaming fluid. However, the pulse is also deformed by the flow velocity profile and the pulse width is broadened at the same time by heat diffusion when it propagates down the stream. As a result, the pulse width tends to be too broad to be useful for measurement of slow flows and the flow rate measurement become inaccurate particularly for measuring flows below certain flow velocity.
The flow measurements apply the calorimetric flow sensors usually are implemented with a heater surrounded by temperature sensitive elements arranged symmetrically downstream and upstream. A moving fluid continuously carries away heat from its surroundings as the fluid is moving along the direction of flow thus changes the temperature distribution around the heater. The temperature difference between upstream and downstream is measured by the temperature sensitive elements. The output signal is commonly fetched using a Wheatstone bridge circuit, in which a pair of downstream and upstream sensing elements comprises two of its four branches. The output signal, which is a measure of temperature difference, is proportional to the flow velocity initially until a high flow velocity is reached where the temperature difference saturates and then decreases at higher flow velocity.
The heater is usually kept at a constant temperature above the ambient temperature. That is, the heater is operated in constant-temperature mode. The heater can also be operated in constant-power mode, in which the power supply to the heater is kept as constant. The ambient temperature is monitored by a reference resistor, which is made of the same material as the heater. A Wheatstone bridge circuit consisting of the heater and the reference resistor can be formed to achieve constant-temperature control.
U.S. Pat. No. 4,501,144 describes a calorimetric flow sensor, which was designed to measure either average gas velocity or mass flow rate through a flow channel. This mass flow sensor consisted of two thermally isolated silicon nitride membranes with a central heating, serpentine-resistor-element grid divided equally between the two bridges (or cantilevers). In addition, two identical thin-film serpentine resistor grids with relatively large temperature coefficients of resistance (TCRs) served as temperature sensors, placed symmetrically with respect to the heater on each microbridge. The sensor and heater grids were made of diffused or (temperature-sensitive) thin-film platinum or permalloy (Ni80Fe20), and were encapsulated in a 0.8˜1.0 micron thick dielectric silicon nitride film, which comprised the suspended microbridges. Anisotropic etching of the silicon substrate (with KOH plus isopropyl alcohol) was used to create an air space pit below the microbridges that was preferably ˜125 micron deep, precisely bounded on the sides by (111) silicon planes, and on the pit bottom and ends of the bridges by the (100) and other planes. The symmetry and effectiveness of the microbridge that is etched undercut was maximized by orienting the longitudinal axis of each bridge at an angle of 45° with respect to the <110> direction in the monocrystalline silicon substrate.
In a U.S. Pat. No. 6,550,324, Mayer et al. disclosed a mass flow sensor. As that shown in FIG. 1B, the flow sensor includes a heating element (4) arranged between two temperature sensors in order to measure the mass flow of a liquid or a gas. The mass flow is determined from the temperature difference of the temperature sensors (5, 6). For the pulse of reducing power consumptions, electric pulses are provided to operate the heating element (4). A further reduction of the power consumption is reached by means of a monitoring circuit (12), which switches the actual measuring section (11) on only if the signals from the temperature sensors (5, 6) fulfill a threshold condition. The pulsed power techniques as discussed above still face the difficulties that the range of measurements and accuracy are limited.
However, the above-mentioned techniques as discussed do not provide a resolution to the major concerns for mass flow sensors. Specifically, for those of ordinary skill in the art there is still a need to provide a mass flow sensor to reduce the heating power consumption and to expand the measurable flow rate range with sufficient accuracy.